BONES AND SCALES 335 



tissue. Such may be termed investment bones 

 (Allostoses, (Jaiipp . 



Finally a third category of bones are the substitution bones 

 (corresponding roughly to the old group of cartilage bones; Auto- 

 stoses, Gaupp). In these the formation of bone has spread into the 

 connective tissue in immediate contact with the cartilage, and as the 

 tissue is formed, room for it is made by the destruction of the pre- 

 viously existing cartilage, which it therefore comes to replace. 



While it is convenient to recognize these three types of bone 

 development, and probably justifiable to interpret them as represent- 

 ing successive steps in the evolution of bone, it must not be supposed 

 that they are absolutely distinct : intermediate forms occur frequently 

 and a single bone of the adult may arise during ontogeny in part 

 according to one type and in part according to another. 



Bony tissue being rigid and inextensible, it is essential to the 

 functions of movement and growth, that it should not be continuous 

 throughout the body. It consequently takes the form of separate 

 bones, the junctions between which are specialized either for move- 

 ment, or for addition of new bony tissue at their margins. Each 

 bone arises by the spreading outwards of bony tissue from one or 

 more centres of ossification. The study of the arrangement and 

 honiology of the various bones constitutes an important part of the 

 science of Comparative Anatomy particularly important for the 

 reason that it is the bony skeleton alone which is as a rule preserved 

 in the fossil remains of Vertebrates belonging to past phases of 

 Evolution. 



It should be borne in mind that a single bony plate in such a 

 part of the skeleton as the skull may represent ossification which has 

 spread out irregularly from the bases of a large number of the original 

 placoid elements. In view of this it will be realized that great 

 caution must be exercised in homologizing apparently similar bones 

 in different groups of the lower Vertebrates. Thus the same name 

 implying homology is commonly given to similar bones in the 

 skull of a Crossopterygian, an Actinopterygian, and a Lung-fish or 

 Amphibian. There is no guarantee of any precise homology in such 

 cases and the student should be on his guard against taking very 

 seriously the nomenclature of such bones as expressing exact and 

 well-determined homology. 



FISH SCALES. In the Fishes, that is in those Gnathostomata 

 in which the skin has not yet become specialized for Respiration 

 (Amphibians), or for protection against desiccation (Reptiles), or for 

 diminishing loss of heat (Birds and Mammals), there is commonly 

 present a coating of dermal bones which most usually take the form 

 of scales. Such scales are in the most general terms simply plates 

 of bone in one or other of its varieties. The development of what is 

 probably the most primitive type the placoid scale has already been 

 dealt with. It need only be added that individual scales, inter- 

 spersed regularly amongst the others, pause in their development, and 



